Can I assign a toggle shortcut to the Cursor Depth feature in User Preferences?
Easy.

Go to hot-key tab in user-preference. (Ctrl Alt U)

Find 3D-view, then go to 3D-view(Global), scroll down and find the 'Add New' inside this hierarchy.

After you press 'Add New', a new hotkey entry with the name 'none' and the key of 'A' will appear.

Click the arrow to the left of it, in the text box where it says 'none', add this:

'wm.context_toggle'

A new box appears, 'Context Attributes'.  add this:

'user_preferences.view.use_mouse_depth_cursor'

Give it a hotkey that does not conflict with any existing hotkey in 3D-View.

Done.

PS: If you have the Pie Menu Editor Addon, you can also add the property into a Pie Menu, which saves hotkeys.  In that case, you have to use this:

'C.user_preferences.view.use_mouse_depth_cursor'

or

'bpy.context.user_preferences.view.use_mouse_depth_cursor'